A polyolefin resin such as a polypropylene resin is excellent in electric properties, mechanical properties, chemical properties, formativeness, hygienic property and recyclability, and accordingly is used mainly for automobiles, electric appliances, packaging materials, daily miscellaneous goods and the like. However, in general, the polyolefin resin has no polar group in the molecular chain thereof, and therefore, unfortunately, it is difficult to apply a coating material or the like to the polyolefin resin or to allow the polyolefin resin to adhere with an adhesive.
Accordingly, it is proposed to use a chlorinated polyolefin resin as a binder component of a coating material or an adhesive when a coating material is applied to a polyolefin resin such as a polypropylene resin, or when a polyolefin resin such as a polypropylene resin is allowed to adhere with an adhesive. However, a chlorinated polyolefin resin generates a harmful substance such as an acidic gas when the chlorinated polyolefin resin is discarded by incineration, and hence, recently a transition to a non-chlorine-based binder resin has been strongly demanded as the environmental concern grows.
In packaging materials, a polyolefin resin is used as, for example, a sealant resin, in such a way that there is used a laminated body obtained by laminating the sealant resin prepared from a polyolefin resin on a substrate.
As a method for producing a laminated body having such a configuration, for example, the following methods have been adopted: a dry lamination method in which an adhesive layer is provided on a substrate, and then a sealant resin film beforehand formed in a film form is bonded to the adhesive layer while the sealant resin film is being pressurized with a heating roll to form a sealant layer; and an extrusion lamination method in which an adhesive layer is provided on a substrate, and then a molten sealant resin is extruded from an extruder on the adhesive layer to form a sealant layer.
The extrusion lamination method has, for example, an advantage such that a sealant resin is not required to be beforehand formed in a film, or an advantage such that even when the adhesive layer is thin, a sufficient adhesive strength is obtained, and hence is suitable for a mass production of laminated bodies at a low cost. The extrusion lamination method using a polyethylene resin as a sealant resin has been widely used.
However, in the adhesion between a substrate and a polypropylene resin, no adhesive having a practical adhesive strength has been available in the extrusion lamination method, and accordingly, when a laminated body is produced by using a polypropylene resin as a sealant layer, the dry lamination method large in number of steps and high in cost has been exclusively adopted.
Examples of the method for providing an adhesive layer on a substrate as described above include a method in which a resin is converted into a state of being a liquid material and the liquid material is applied. For the purpose of converting a resin into a liquid material, for example, the following methods are used: (a) a method in which a resin is used in a molten state; (b) a method in which a resin is used as dissolved or dispersed in an organic solvent; and (c) a method in which a resin is used as dispersed in an aqueous medium. However, the method of (a) is unfortunately limited in applications, or unfortunately finds difficulty in forming a thin film depending on the balance with the melt viscosity of the resin. Accordingly, although the methods of (b) and (c) are generally understood to be preferable, nowadays, the use of organic solvents tends to be restricted from the viewpoints of environmental protection, resources saving, hazardous materials regulation due to the Fire Services Act or the like, and workplace environment improvement, and thus, the method of (c), namely the method in which a resin is used as dispersed in an aqueous medium is regarded as most preferable.
When an aqueous dispersion is prepared from a resin, a surfactant is generally used in order to promote the dispersion of the resin. However, in general, a surfactant is nonvolatile, and even when a coating film obtained from an aqueous dispersion prepared from a resin is dried, the surfactant remains in the coating film; consequently, when an amount used of a surfactant is large, unfortunately the water resistance or the chemical resistance of the coating film is remarkably degraded, the tendency for the surfactant to bleed out from the coating film is enhanced to be unfavorable from environmental and hygienic points of view, and moreover, the performances of the coating film are also liable to be varied with time.
From such a background as described above, the development of modified polyolefin resins obtained by modifying the polyolefin resins with acids or the like, as nonchlorine-based binder resins, has been performed, and the preparation of an aqueous dispersion of an acid-modified polyolefin resin without using any surfactant has been investigated. For example, Patent Literature 1 discloses a technique for obtaining an aqueous acid-modified polyolefin resin dispersion substantially without using any surfactant.